feat: add `SymM` monad (#11788)

This PR introduces `SymM`, a new monad for implementing symbolic
simulators (e.g., verification condition generators) in Lean. The monad
addresses performance issues found in symbolic simulators built on top
of user-facing tactics like `apply` and `intros`.

**Key features:**
- Goals are represented by `Grind.Goal` objects, enabling incremental
hypothesis processing
- No `revert` or `clear` operations, allowing O(1) local context checks
instead of O(n log n)
- Carries `GrindM` state across goals to avoid reprocessing shared
hypotheses
- Provides `mkGoal` for creating new goals within the monad

This is the foundational infrastructure for `SymM`. Future PRs will add
operations like `intro`, `apply`, and the optimized definitional
equality test.